The Impact of Deuteration on the Infrared Spectra of Interstellar Polycyclic Aromatic Hydrocarbons

Abstract

In this paper, we present the results of a computational study of the effects of a low absolute level of deuteration (similar to5%) on the observed mid-IR spectrum of the interstellar polycyclic aromatic hydrocarbon (PAH) population. The analyses are based on a global average spectrum, including all the possible singly deuterated isomers of an ensemble of partially superhydrogenated PAH species (i.e., H-n-PAHs) in both neutral and cationic forms. The distinctive spectroscopic properties considered in this paper include three classes of D-involved vibrational modes: (1) C-D in-plane bending features in the 9.5-12 mum region, (2) C-D out-of-plane bending features in the 14.5-17.9 mum region, and (3) aromatic and aliphatic C-D stretching features in the 4.4-4.7 mum region. In addition, D substitution also has the potential to alter the appearance of the C-H out-of-plane bending features in the 11-14 mum region by disrupting the adjacency of the H atoms on the PAH edge rings. The spectroscopic impacts of a low level of deuteration in the interstellar PAH population are considered, and their implications for the detailed appearance of the astronomical mid-IR PAH emission spectrum is explored. Overall, the results of this study indicate that the C-D stretching features offer the greatest prospects for the detection of deuterated interstellar PAHs. Since these features are effectively obscured by telluric CO2 absorptions, the information presented herein should be useful in the planning and design of future space-based IR observing platforms.